1. Field of the Invention
This invention relates to a particle beam apparatus, and more particularly to a particle beam apparatus with an energy filter.
2. Discussion of Relevant Art
Particle beam apparatuses in the form of transmission electron microscopes with energy filters are known, for example, from U.S. Pat. Nos. 4,740,704, 4,760,261 and 5,449,914. The energy filters described in these documents are dispersive, that is, a charged particle entering the filter undergoes, on passing through the filter, a deflection which depends on the particle energy. The filter described in U.S. Pat. No. 4,740,704 is used by the inventors employer' in the transmission electron microscope 912 Omega manufactured and sold by LEO Elektronenmikroskopie GmbH. In the 912 Omega, the filter is arranged in the imaging beam path between the specimen to be investigated and the projection screen or the camera on which the specimen is electron-optically imaged. With such an energy filter on the imaging side, the energy loss which the particles have undergone in the specimen can be analyzed. At the same time, the imaging errors which depend on the energy, the chromatic aberrations, are reduced in the imaging beam path, since only particles with a reduced energy bandwidth contribute to the imaging.
For the correction of chromatic imaging errors, both in scanning electron microscopes and also in transmission electron microscopes, it is known from U.S. Pat. No. 5,319,207 to provide a mirror corrector in the illumination beam path between the electron source and the specimen to be investigated. The mirror corrector consists of a magnetic beam deflector and an electrostatic mirror which images into each other the two planes of symmetry within the magnetic beam deflector. Although the beam deflector has dispersive properties, the corrector is non-dispersive overall, that is, particles entering the corrector undergo, after passing completely through the corrector, no deflection which is dependent on the particle energy.
Such correctors are however relatively expensive and up to now have not been commercially offered.
As an alternative to a corrector, it is known from an article by H. Rose in Optik (Optics), Vol. 85 (No. 3), pp. 95-98 (1990), to provide an energy filter in the illuminating beam path of a transmission electron microscope. The energy filtering which is effected permits at least the energy-dependent errors to be reduced, because of the small energy bandwidth of the particles that contribute to subsequent imaging. Although here also the filter has dispersive elements for the splitting of the particle beam according to energy, the filter is overall free from dispersion, so that the particles entering the filter again undergo, after completely passing through the filter, no deflection which depends on energy. The freedom from dispersion of the whole filter is attained in that the filter is symmetrical about a midplane, and the dispersion in both of the mutually symmetrical filter portions is exactly opposed. This freedom of the filter from dispersion insures that small voltage fluctuations at the filter do not lead to a drift of the beam behind the filter. Dispersion-free filters however have the disadvantage that the dispersion that can be attained in the energy selection plane, in which the energy selection takes place by means of a slit diaphragm, is relatively small. And since the dispersion is in general dependent on the particle energy and decreases with increasing particle energy, the particle energy within the filter has to be relatively low when high energy sharpness is to be attained. In the article, the starting point was a particle energy of 3 keV, and in later work by H. Rose a significant energy region of 3-5 keV was specified. At low particle energies within the filter, however, a broadening of the energy bandwidth results because of the so-called Boersch effect. Since the Boersch effect has significant effects particularly in intermediate images of the particle source within the filter, because of the higher particle density in such intermediate images, the use was already proposed by H. Rose of a filter with exclusively astigmatic intermediate images within the filter.
Furthermore, a raster electron microscope with a dispersive energy filter between the source and the objective is known from Japanese Patent JP 62-93848. In the system described there, the filter is however only used for the production of a relative signal, so that the negative influence of the noise of the electron source on the subsequently produced picture can be eliminated by quotient formation between the actual secondary electron measurement signal and the relative signal.